[a11y]: Progress indicator website docs

[mbgower]

Progress indicator component is missing most parts of the Usage template, and as a result is lacking many accessibility considerations

We completed a quick video of the Progress Indicator interaction in storybook, which I highly recommend you watch. Password: wHhv2TW9 (only the first half until about the 11 minute mark is progress indicator).
Most of the issues we uncovered are not so much development bugs as they are challenges with the basic progress indicator design -- and the lack of sufficient real-world context in the documentation.
For that reason, I have opened the issue against the carbon-website repo.

I have attempted to capture the key points as comments in PDF exports of the Usage and Style pages, which should be reviewed. The key points are:

• the normal template for the Usage page has not been followed. It lacks sections on Variants, Formatting (including Anatomy and Placement), Content and Behaviours. The storybook implementation shows that variants and differences in behaviour exist. If they were addressed in the documentation, I believe a number of accessibility questions or problems would be addressed
• the actual visual design of the indicators seems problematic, both as a strictly non-interactive element and as a complex interactive component
• there are very specific accessibility considerations around flow, interaction and programmatic indicators of state/status which are difficult to fully discuss without the above information. I am happy to elaborate on any of the brief comments I make in the video.
• I would consider making the progress indicator only 1 tabstop, like a tablist. That will not only make it faster to use, but also make it potentially less confusion.
• I don't understand to what degree the progress indicator 'owns' a section of the page, like a tablist. I don't think that's the case; it is more analogous to a shopping cart. But I'd like to explore that.
• there is no apparent visual difference between an interactive and non-interactive progress indicator. I find that problematic.

A couple of quick notes on implementation in storybook

• there seems to be no programmatic equivalent to those 'current', 'complete', 'incomplete' symbols
• :: before psuedo code is used for the symbols; that will lead to accessibility problems

[sstrubberg]

Closing in favor of tracking via carbon-design-system/carbon#13521